Your Best Subject Line In an Email? i will not promote

I'm starting to do outreach to small businesses and potential customers to validate my start-up idea. Some subject lines are is getting responses and one works about 30% of the time but about half of respondents from that subject line are less than thrilled. I am curious what y'all are using to get more people to respond to you and how positive those responses are? I realize some of these are likely to be specific to your start up but hearing about how you are approaching things is a great way for all of us to get better!